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Chapter 7. Memory: Introduction > Paging and Swapping

7.4. Paging and Swapping

When a process references a page on disk, the page must be paged in, which could cause other pages to page out again. VMM is constantly working in the background, stealing frames that have not been recently referenced using the page replacement algorithm. It also helps detect thrashing, which can occur when memory is extremely low and pages are constantly being paged in and out to support processing. VMM actually has a memory load control algorithm, which can detect whether the system is thrashing and actually tries to remedy the situation. Unabashed thrashing can literally cause a system to come to a standstill, as the kernel becomes so concerned with making room for pages that it just can't do anything productive.

What about swapping? Although the terms are often used interchangeably, there is a subtle difference between paging and swapping. As we've discussed, with paging, only parts of the process are moved back and forth between disk and RAM. When swapping occurs, you are moving entire processes back and forth. For this to happen, AIX would need to suspend the entire process before moving it to paging space. It could then only continue to process when the process was swapped back into RAM at a later time. The difference that is not subtle is this: while paging is often okay, swapping is a very bad thing.


  

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